This invention relates to methods of and apparatus for controlling combustion in furnaces, and more particularly it is concerned with a combustion control method suitable for maintaining the thermal efficiency of a plant at a highest possible level while meeting the requirements of minimizing the amounts of the oxides of nitrogen produced and the unburned fuel remaining in the ash which are necessary in operating the plant to avoid air pollution.
Heretofore, several methods have been available for controlling combustion taking place in a furnace. In one method known in the art, rays of light emitted by the flames produced by combustion in the furnace are monitored and the ratio of the fuel volume to the air volume supplied to the furnace is controlled in such a manner that the spectrographic intensity of the light is maximized to obtain thermal energy with a maximum efficiency, as disclosed in Japanese Patent Application Laid-Open No. 100224/81 entitled "Method and Apparatus for Controlling Combustion". In another method known in the art, the volume of air supplied to the furnace for combustion is efficiently controlled in accordance with the volume of light emitted by the flames of combustion, to thereby optimize the volume of supplied air, as disclosed in Japanese Patent Application Laid-Open No. 151814/81 entitled "Apparatus for Controlling the Volume of Supplied Air for Combustion". These methods are considered to have effect in maximizing combustion efficiency, but they are unable to effect combustion control in such a manner that the heat absorption factor of the boiler is maximized while achieving the stabilization of combustion.
Meanwhile, no closed-loop control methods have ever been employed for controlling the volume of the nitrogen oxides in the furnace. The reason why such methods have not been adopted is because of the inability to accurately measure the volume of the oxides of nitrogen produced in the furnace as the current state of the art makes it impossible to determine the volume of fuel and air to be controlled to effect control of the volume of the oxides of nitrogen in the furnace. Thus, it has hitherto been usual practice to effect open-loop control of the volume of the oxides of nitrogen by monitoring the value of the oxides of nitrogen sensed at the outlet of the furnace after deciding the volumes of fuel and air conforming to a load in accordance with a program control. Thus, it has hitherto been impossible to control the volume of the oxides of nitrogen satisfactorily in plants where the character of fuel undergoes a change due to a variation in the type of coal burned or the volume of coal supplied show fluctuations.
With regard to the stability of combustion, it has hitherto been the usual practice to rely on the use of a television camera mounted to a peep hole at the top of the furnace for obtaining an image of the flames produced in the furnace by combustion which is shown on monitor television receivers to enable the operator to assess the condition of combustion to determine its stability. This method requires the use of operators qualified to do the job and having long experiences in fulfilling the duties, and suffers the disadvantage that the results achieved may vary from one operator to another because the assessments might possibly be affected by individual propensity.
As noted hereinabove, the current state of the art concerning combustion control makes it impossible to effect control of combustion in plants, particularly those plants in which the character of fuel undergoes a change or the volume of supplied fuel shows fluctuations, in such a manner that the combustion is stabilized and the thermal efficiency is maintained at the highest level possible while the requirements to keep the parameters important in operating a plant, such as the amounts of the oxides of nitrogen produced and the unburned fuel remaining in the ash, at desired levels are met.